Systems are made for administering to patients pacing pulses externally. Advantageously, external defibrillators may also administer such external pacing. A common electrode used to connect a heart pacer to a patient incorporates an electrically conductive, impedance-decreasing gel disposed between a flexible conductive plate and the patient's skin. The gel ensures good electrical contact between the patient and the conductive plate, and adheres the electrode to the patient's skin. During pacing, pulses are generated by the heart pacer and applied through the electrodes and into the patient. Typical pacing equipment will commonly deliver a pacing pulse having an amplitude of up to 300 volts, and a maximum current of about 0.2 amps. Such a pacing pulse may be applied to a patient up to 170 times a minute, for periods as long as 24 hours.
FIG. 1 is a diagram of a pacing waveform 10 currently used for the transcutaneous pacing of a patient. Pacing waveform 10 is shown on a graph having a horizontal axis of time, and a vertical axis of current (in milliamps) applied to the patient. Prior art pacing waveform 10 comprises a positive stimulating pacing pulse 12 having an amplitude 14 and a duration t1. After application of the positive stimulating pulse, the amplitude of the waveform falls to at or near 0 for duration t2. The period between application of the positive stimulating pulses is the sum of duration t1 and t2.
Application of pacing waveform 10 generates an excess of positive charge flowing through the electrodes. The excess charge causes hydrolysis of the electrode gel, producing hydrogen and oxygen between the electrode and the patient's skin. During long-term transcutaneous pacing, the generation of hydrogen and oxygen causes the electrode impedance to increase and the electrode pH to change.
A problem arises when such transcutaneous pacing is long-term. The changes of an electrode's impedance and pH are dramatic. The hydrogen and oxygen gas, which tend to accumulate between the patient's skin and the flexible conductive plate, and produces two primary undesirable effects. First, the accumulation of the gases generally decreases the conductivity between the electrode and the patient. As the impedance of the electrodes increases, the pacer is forced to compensate by applying a higher voltage to produce a suitable pacing current. Generally, the impedance of the electrodes may reach such a high value that the pacer is unable to generate a sufficient voltage to apply a pacing pulse. At this point, many pacers may stop pacing, and instead generate a “leads off” alarm, on the erroneous determination that the high resistance is caused by one of the electrodes having fallen off the patient.
Second, the gas has a tendency to accumulate in pockets, causing the current density in areas of the electrode to increase. The bubbles tend to insulate the conductive plate from the patient, reducing the surface area of the electrode in contact with the patient. If the density of current flow increases in the areas remaining in contact with the patient, patient discomfort may result. If the current density increases even further, burning of the patient's skin may result. The increasing current density problem is exacerbated with electrodes designed for pediatric use. Pediatric electrodes tend to be smaller and have a smaller conductive surface, yet have current flows comparable to electrodes used for adults.
As hydrolysis occurs during long-term pacing, the pH of the electrode also has a tendency to change. The formation of hydrogen and oxygen bubbles within the electrode cause the gel of one electrode to become more acidic, and the gel of the other electrode to become more basic. For children or patients with sensitive skin, the change in electrode pH can become highly irritating to the dermic layer.
One approach to minimizing the buildup of hydrogen and oxygen gas has been to construct the electrodes in a manner that minimizes the amount of gas that may accumulate. For example, U.S. Pat. No. 5,456,710 entitled ‘VENTED ELECTRODE” discloses an electrode construction that allows hydrogen and oxygen buildup within an electrode to pass through a gas-permeable layer and escape from beneath the electrode. The gas generated by hydrolysis can therefore vent to the environment before accumulating and causing the impedance or pH of the electrode to change.
While the accumulation of hydrogen and oxygen in the electrodes may be prevented by appropriately constructing the electrodes, such a solution is only applicable in limited circumstances. The majority of pacing performed today uses traditional, non-vented conducting pads. For those situations where non-vented electrodes are being used, it would be advantageous to find an alternative technique to minimize the hydrolysis of the electrode gel such that the impedance and pH of the electrodes would remain relatively constant over an extended period of time.